As the dimensions of particles decrease, their surface composition and surface area become leading factors in determining end-use viability. Effective ways to encapsulate and functionalize the surfaces of fine particles are becoming more important as uses of such particles become more apparent. For example, after surface modification carbon nanotubes show improved dispersion and bonding with a polymer matrix, making them potential strengthening fillers in light-weight polymer composites. [Eitan, A., Jiang, K. Y., Dukes, D., Andrews, R. & Schadler, L. S. Surface modification of multiwalled carbon nanotubes: Toward the tailoring of the interface in polymer composites. Chem. Mater. 15, 3198-3201 (2003); Gong, X., Liu, J., Baskaran, S., Voise, R. D. & Young, J. S. Surfactant-assisted processing of carbon nanotube/polymer composites. Chem. Mater. 12, 1049-1052 (2000); Mitchell, C. A., Bahr, J. L., Arepalli, S., Tour, J. M. & Krishnamoorti, R. Dispersion of functionalized carbon nanotubes in polystyrene. Macromolecules 35, 8825-8830 (2002); and Shaffer, M. S. P. & Windle, A. H. Fabrication and characterization of carbon nanotube/poly(vinyl alcohol) composites. Adv. Mater. 11, 937 (1999).] Or, for example, drug particles and drug-loaded microspheres encapsulated with pH-sensitive polymers, which provide targeted release based on the pH of the intended environment. [Uhrich, K. E., Cannizzaro, S. M., Langer, R. S. & Shakesheff, K. M. Polymeric systems for controlled drug release. Chem. Rev. 99, 3181-3198 (1999); Schmid, S., Wahl, M. A. & Schmidt, P. C. Enteric coating of ibuprofen crystals using modified methacrylate copolymers. Drugs Made in Germany 44, 12-19 (2001); Haining, W. N. et al. pH-triggered microparticles for peptide vaccination. J. Immunol. 173, 2578-2585 (2004); and Perumal, D. Microencapsulation of ibuprofen and Eudragit® RS 100 by the emulsion solvent diffusion technique. Int. J. Pharm. 218, 1-11 (2001). Biodegradable poly(alkyl cyanoacrylate) nanoparticles loaded with therapeutic agents and coated with polysorbate enable drug delivery to the brain. Kreuter, J. Nanoparticulate systems for brain delivery of drugs. Adv. Drug Deliv. Rev. 47, 65-81 (2001); Kreuter, J. Influence of the surface properties on nanoparticle-mediated transport of drugs to the brain. J. Nanosci. Nanotech. 4, 484-488 (2004); and Moghimi, S. M., Hunter, A. C. & Murray, J. C. Long-circulating and target-specific nanoparticles: Theory to practice. Pharmacol. Rev. 53, 283-318 (2001).] It has also been shown that magnetic nanoparticles coated with dextran give enhanced sensitivity in magnetic resonance imaging. [Chouly, C., Pouliquen, D., Lucet, I., Jeune, J. J. & Jallet, P. Development of superparamagnetic nanoparticles for MRI: Effect of particle size, charge and surface nature on biodistribution. J. Microencapsulation 13, 245-255 (1996). Semiconductor nanocrystals coated with amphiphilic polymer shells and other biological interfaces offer the potential for single quantum dot tracking and sensing in cell biology. Michalet, X. et al. Quantum dots for live cells, in vivo imaging, and diagnostics. Science 307, 538-544 (2005); and Medintz, I. L., Uyeda, H. T., Goldman, E. R. & Mattoussi, H. Quantum dot bioconjugates for imaging, labeling and sensing. Nature Mater. 4, 435-446 (2005).] In sum, microparticles and nanoparticles are useful in a wide range of applications in biotechnology, pharmaceutics, optics, electronics, aviation, and aerospace. The surface properties of the particles play a crucial role in determining the overall function and performance of a particle-based device; in many cases, functional polymers are used to define the final nature of the particle surface. This fact underscores the importance of strategies for optimal encapsulation, functionalization and modification with polymeric materials of the surfaces of microparticles and nanoparticles.
Current strategies for encapsulating particles with polymers are mainly liquid-based protocols that rely on applying a polymer coating solution onto a particle surface with the subsequent removal of solvent. For example, spray coating of a polymer solution onto a fluidized bed of particles allows a polymer coating to form around each particle as the solvent evaporates. [Guignon, B., Duquenoy, A. & Dumoulin, E. D. Fluid bed encapsulation of particles: principles and practice. Drying Technol. 20, 419-447 (2002).] In another approach, an emulsification-solvent evaporation technique, utilizing a single or double emulsion of wet polymer capsules around core microdroplets, enables a polymer to precipitate around each core particle upon solvent drying. [Rosca, I. D., Watari, F. & Uo, M. Microparticle formation and its mechanism in single and double emulsion solvent evaporation. J. Control. Release 99, 271-280 (2004).] Alternatively, a layer-by-layer adsorption of polyelectrolyte multilayers with alternating charge can be performed on particles in a colloidal suspension. [Caruso, F., Caruso, R. A. & Möhwald, H. Nanoengineering of inorganic and hybrid hollow spheres by colloidal templating. Science 282, 1111-1114 (1998); and Sukhorukov, G. B. et al. Stepwise polyelectrolyte assembly on particle surfaces: a novel approach to colloid design. Polym. Adv. Technol. 9, 759-767 (1998).] Among dry encapsulation methods, plasma enhanced chemical vapor deposition (PECVD) is demonstrated to provide conformal solid coatings with polymer-like compositions under a low pressure gaseous environment. Using vibration or fluidization to agitate the particles during deposition, coatings have been made on drug microcrystals, ceramic nanoparticles and carbon nanotubes. [Susut, C. & Timmons, R. B. Plasma enhanced chemical vapor depositions to encapsulate crystals in thin polymeric films: a new approach to controlling drug release rates. Int. J. Pharm. 288, 253-261 (2005); Vollath, D. & Szabó, D. V. Coated nanoparticles: a new way to improved nanocomposites. J. Nanoparticle Res. 1, 235-242 (1999); Lamparth, I., Szabó, D. V. & Vollath, D. Ceramic nanoparticles coated with polymers based on acrylic derivatives. Macromol. Symp. 181, 107-112 (2002); Shi, D. et al. Uniform deposition of ultrathin polymer films on the surfaces of Al2O3 nanoparticles by a plasma treatment. Appl. Phys. Lett. 78, 1243-1245 (2001); and Shi, D. et al. Plasma deposition of ultrathin polymer films on carbon nanotubes. Appl. Phys. Lett. 81, 5216-5218 (2002).]
George et al. have disclosed particles having an ultrathin, conformational coating, made using atomic layer deposition methods. These coated particles are useful as fillers for electronic packaging applications, for making ceramic or cermet parts, as supported catalysts, as well as other applications. However, these methods are limited to depositing inorganic films. [George et al. U.S. Pat. No. 6,613,383, herein incorporated by reference; George et al. U.S. Pat. No. 6,913,827, herein incorporated by reference; and George et al. United States patent application Publication No. U.S. 2003/0026989, herein incorporated by reference].